One known ink cartridge is featured to have a rigid case and a soft ink bag inside the case. For example, Chinese patent number CN1319502 discloses an ink cartridge which has an ink bag inside a sealed rigid case. Applying air pressure into the case presses the ink bag to supply ink to the print head. The case has a bottom part and a top part. After the ink bag is placed in the bottom part, the top part covers the bottom part and the case is then sealed.
Usually, the print head has a certain degree of suction to suck the ink from a level below the print head. However, if the ink level is too low, the print head cannot suck the ink by itself. An ink supply system is then needed. The ink cartridge disclosed in CN1319502 is suitable for use in this type of printer. The print head does not need any suction. An air pump provides constant air pressure to the case and induces pressure onto the ink bag to overcome the deficiency of the print head suction.
However, the known ink cartridge is very complex in structure and it thus is, expensive. It requires the printer to be equipped with an air pump. This increases the cost of the printer. Moreover, the ink cartridge cannot be refilled by ordinary users. The used cartridges need to be sent to professionals to refill. Further, air has a greater expansion coefficient than liquid ink. Therefore, if the ink cartridge is not used for a long period of time, the temperature change in the environment may generate pressure on the ink bag that pushes the ink overflow from the ink outlet, causing contamination.